Seven years after he wrote a book about impeachment, then-U.S. Supreme Court Chief Justice William Rehnquist presided over one: the trial of President Bill Clinton in 1999, which resulted in acquittal. The book, and the experience, made Rehnquist an unparalleled expert on impeachment.

As the U.S. House moves forward with its impeachment inquiry of President Donald Trump, one of Rehnquist's former clerks, now-Chief Justice John Roberts Jr., would be poised to preside over any trial in the U.S. Senate. Of course, only if the process gets to that point.

Roberts and Trump have a complicated history laced with rebukes from both sides.

Roberts portrays himself as an institutionalist, protective more of how the court—and the judiciary—is viewed than how he is seen. Last year, Roberts issued a rare rebuke of Trump after one of his criticisms of a court ruling by an "Obama judge." The chief said: "We do not have Obama judges or Trump judges, Bush judges or Clinton judges. What we have is an extraordinary group of dedicated judges doing their level best to do equal right to those appearing before them."

The chief justice has long painted himself as a nonpartisan leader of the court, eschewing any notion that the justices take sides based on which president appointed them to the high court.

"When you live in a polarized political environment, people tend to see everything in those terms," Robert said in remarks Tuesday in New York. "That's not how we at the court function and the results in our cases do not suggest otherwise."

Trump has called Roberts a "nightmare" for conservatives and an "absolute disaster," criticism tied to the chief justice's rulings that upheld the signature Obama legislation the Affordable Care Act. Trump has said he would ask the Supreme Court to intervene if the House moved to impeach him. But that's not how it works—there is no appeal.

Rehnquist's 1992 book "Grand Inquests" chronicled the impeachment of Supreme Court Justice Samuel Chase in 1805 and President Andrew Johnson in 1868. Both ended in acquittals after failing to garner a two-thirds majority in the Senate as required by the Constitution.

The outcomes, in Rehnquist's view, established the important principle that impeachment must not be used lightly by Congress as a weapon to punish the other branches for policy differences.

"In retrospect, one cannot imagine anyone better qualified than Chief Justice Rehnquist to preside over the Clinton impeachment trial," Michael Gerhardt, a professor at University of North Carolina School of Law who's also written about impeachment, told The National Law Journal in a 2017 interview.

Rehnquist, who was an official in the Nixon Justice Department before joining the Supreme Court, was scrupulously nonpartisan and low key in running the Clinton trial. After it was over, Rehnquist borrowed a line from Gilbert and Sullivan in describing his role: "I did nothing in particular, and I did it very well." In February 1999, Clinton was acquitted in the U.S. Senate.

Rehnquist died in 2005 and his papers reside at the Hoover Institution in California. His impeachment files are closed to the public for the lifetime of other justices who served on the court with him.

Rehnquist's observations about impeachment in the book and other sources resonate in the current debate, Gerhardt said in the 2017 interview. "The lesson to be learned from Rehnquist that can be applied to Trump is to remember impeachment is not supposed to be a partisan tool. It is a last resort to be used for serious abuses of power or breaches of trust."

Some excerpts from Rehnquist's writings on impeachment: 

Congressional supremacy: "Had these two trials resulted in conviction rather than acquittal, the nation would have moved closer to a regime of the sort of congressional supremacy that was dreaded by men such as James Madison, the Father of the Constitution. … We twice escaped from a precedent that would indeed have given us a Presidency, and a Court, subject to 'tenure during the pleasure of the Senate.'"

Indictable offense: "All members of the Judiciary Committee [investigating Richard Nixon] appear to have rejected the view that a constitutional 'high crime or misdemeanor' must be an indictable offense under the criminal law."

Overreaching and bullying: "One need only note the way in which the framers arranged the test of the United States Constitution to realize that they were concerned about the separation of powers within the new federal government they were creating. … The framers were particularly concerned about the possibility of overreaching and bullying by the legislative branch—Congress—against the other branches."

Answerable to the country: "The importance of the acquittal [of President Andrew Johnson] can hardly be overstated. With respect to the Chief Executive, it has meant that as to the policies he sought to pursue, he would be answerable only to the country as a whole in the quadrennial Presidential elections, and not to Congress through the process of impeachment."

Obstruction of justice: "The counts relating to the obstruction of justice and to the unlawful use of executive power [by President Nixon] were of the kind that would surely have justified removal from office."